A first-order cosmological phase transition can only occur in the presence of a scalar effective potential
barrier separating the symmetry-broken and -unbroken vacua.
Although the probability of tunneling from the metastable minimum to the stable one via the instantons is very tiny,
the decay of the false vacuum can proceed through thermal fluctuations which help overcome the potential barrier.
The tunneling rate per unit volume and time element is approximately given by [1]}, [2]}
\(\Gamma (T)=A(T)e^{-S_{3}/T},\) 
